Rack extension



FIG. 1 is a top, front, and right side perspective view of a rack extension showing my new design;

FIG. 2 is a reduced left side elevational view thereof;

FIG. 3 is a reduced top plan view thereof;

FIG. 4 is a reduced bottom plan view thereof; and,

FIG. 5 is a reduced front elevational view thereof. 

The ornamental design for a rack extension, as shown and described. 